1. Field of the Invention
The invention relates to electronic circuits, and in particular to a power-efficient current mirror with high output impedance and a wide output voltage range.
2. Related Art
A current mirror is used to duplicate a reference current in an integrated circuit (IC) for use in a different portion(s) of the IC. By providing this duplicate current, the current mirror can minimize the effects of the circuit operation on the reference current source.
FIG. 1 shows a conventional current mirror 100 that includes a reference current source CS11, an output terminal 101, a reference transistor M11, and an output transistor M12. Current source CS11 and transistor M11 are connected in series between a supply voltage VDD and ground, while transistor M12 is connected between output terminal 101 and ground. The gates of transistors M11 and M12 are connected, and the gate of transistor M11 is connected to its drain (i.e., transistor M11 is diode-connected).
To provide proper current mirroring, transistors M11 and M12 must be operating in saturation to ensure that voltage changes at output terminal 101 do not affect the value of output current I_OUT. As is known in the art, a transistor operates in its saturated region when its drain-source voltage Vds is at least as great as its gate (gate-source) voltage Vgs minus its threshold voltage Vt (i.e., the voltage at which the inversion layer is formed). The minimum value of the source-drain voltage Vds that satisfies this relationship is termed the saturation (or overdrive) voltage of the transistor, and can be expressed as follows:Vdsat=Vgs−Vt  (1)where Vdsat is the saturation voltage of the transistor.
Since transistor M11 is diode-connected, its drain-source voltage is guaranteed to be larger than its gate voltage, and so transistor M11 is in saturation. Therefore, when current source CS11 supplies a reference current I_REF to transistor M11, the voltage drop (Vds) across transistor M11 required to sink current I_REF is a saturation voltage Vdsat(11).
Then, using Equation 1, the gate voltage of transistor M11 can be determined as follows:Vgs(11)=Vdsat(11)+Vt(11)  (2)where Vgs(11) is the gate-source voltage of transistor M11, and Vt(11) is the threshold voltage of transistor M11.
Because the gates of transistors M11 and M12 are connected, this gate-source voltage is also applied to transistor M12 (i.e., Vgs(12)=Vgs(11)). If transistors M11 and M12 are matched (i.e., the transistors have the same electrical characteristics, such as threshold voltage and saturation voltage), this gate-source voltage causes transistor M12 to sink a current I_OUT that is equal to reference current I_REF. In this manner, reference current I_REF can be mirrored to any circuit coupled to output terminal 101 of current mirror 100.
The minimum output voltage of current mirror 100 at output terminal 101 is equal to the minimum voltage drop across output transistor M12 before it falls out of saturation—i.e., saturation voltage Vdsat(12). Once transistor M12 is not operating in its saturated region, voltage changes at output terminal 101 can affect the current flow through transistor M12, thereby defeating the purpose of current mirror 100. Current mirror 100 beneficially provides a relatively large output voltage range (swing), since it allows proper current mirror operation to occur down to saturation voltage Vdsat(12).
However, because the output of current mirror 100 is at the drain of transistor M12, the output impedance of current mirror 100 is rather low. As is known in the art, the output impedance Rout(100) of current mirror 100 is given by the following:Rout(100)=(λ(12)*I—OUT)−1  (3)where λ(12) is the channel length modulation parameter for transistor M12. Note that this output impedance is simply the output impedance Ro(12) of transistor M12.
To provide a current mirror that has an improved output impedance, negative feedback is sometimes used. For example, FIG. 2 shows a conventional Wilson current mirror 200 that includes a reference current source CS21, an output terminal 201, a reference transistor M21, a control transistor M22, and an output transistor M23. Current source CS21 and transistor M21 are connected in series between a supply voltage VDD and ground, while transistors M22 and M23 are connected in series between output terminal 201 and ground.
Note that, in Wilson current mirror 200, output transistor M23 is diode-connected, rather than reference transistor M21. This creates a negative feedback loop, between the source of control transistor M22 and the gate of reference transistor M21, that holds the output current I_OUT equal to reference current I_REF even if the output voltage (i.e., the voltage at output terminal 201) varies.
For example, an increase in the voltage at output terminal 201 increases the drain voltage of transistor M22, and will therefore attempt to increase the current flow through transistor M22, which in turn would try to force the gate voltage of transistor M23 to increase. This increased gate voltage would also be provided to transistor M21. However, since reference current I_REF is constant, the drain voltage of transistor M21 must then decrease. As a result, the gate voltage of transistor M22 is decreases, thereby maintaining output current I_OUT at a level equal to reference current I_REF.
As is known in the art, the output impedance Rout(22) of Wilson current mirror 200 is given by the following:Rout(200)≈Ro(22)(2+—gm(21)Ro21))  (4)where Ro(22) is the output impedance of transistor 22, gm(21) is the transconductance of transistor M21, and Ro(21) is the output impedance of transistor M21. Thus, the negative feedback loop of Wilson current mirror 200 results in an output impedance that is much greater than the output impedance of transistor M22 by itself.
However, this increased output impedance comes at the cost of reduced output voltage swing (range). As mentioned above, the output voltage of current mirror 100 shown in FIG. 1 can go all the way down to the minimum voltage drop across output transistor M12—i.e., saturation voltage Vdsat(12). By contrast, the minimum output voltage of Wilson current mirror 200 is much higher, resulting in a lesser net output voltage range.
In particular, using Equation (2) above, the gate voltage of transistor M23 is given by:Vgs(23)=Vdsat(23)+Vt(23)  (5)where Vgs(23), Vdsat(23), and Vt(23) are the gate, saturation, and threshold voltages, respectively, of transistor M23. Since transistor M23 is diode-connected, this is also the drain voltage of transistor M23, and the source voltage of transistor M22.
Thus, the minimum output voltage Vo(min) of Wilson current mirror 200 is equal to this gate voltage plus the voltage drop across transistor M23, as shown by the following:Vo(min)=Vdsat(23)+Vt(23)+Vdsat(22)  (6)where Vdsat(22) is the saturation voltage of transistor M22. Transistors M21, M22, and M23 will typically be matched, so that the minimum output voltage Vo(min) for Wilson current mirror 200 given in Equation 6 resolves to:Vo(min)=Vt+2Vdsat  (7)where Vdsat and Vt are the saturation voltage and threshold voltage, respectively, of both transistors M22 and M23 (and transistor M21). Thus, the improved output impedance of Wilson current mirror 200 comes at the expense of reduced output voltage swing, in comparison to current mirror 100.
To provide improved output voltage range while maintaining high output impedance, some current mirror circuits combine a cascoded output with multiple control branches. For instance, FIG. 3 shows a conventional wide-swing cascode current mirror 300 that includes current sources CS31 and CS32 (both providing a reference current Io1), an output terminal 301, and transistors M31, M32, M33, M34, M35, and M36. Current source CS31, transistor M31, and transistor M32 are connected in series between supply voltage VDD and ground to form a first control branch. Current source CS32, transistor M33, and transistor M34 are connected in series between supply voltage VDD and ground to form a second control branch. Transistors M35 and M36 are connected in series between output terminal 301 and ground to form a cascode output branch.
The current mirroring operation of cascode current mirror 300 begins with transistor M33, which is coupled to receive reference current Io1 from current source CS32. Because it is diode-connected, transistor M33 is in saturation and sinks reference current Io1. Transistor M34, which is gate-coupled to the gate of transistor M33, is sized to also sink reference current Io1, but operate in the linear region, as described in greater detail below.
Meanwhile, the gate of transistor M33 is also connected to the gates of transistors M31 and M35. Transistors M31 and M35 are matched to transistor M33, and therefore sink the same current Io1 (from current source CS31 and as output current I_OUT, respectively) in response to the gate voltage from transistor M33.
Finally, transistor M32 is gate-coupled to the drain of transistor M31 and the gate of transistor M36. Since transistor M31 is operating in saturation, transistor M32 is essentially diode-connected, and also operates in saturation to sink current Io1 from transistor M31. Transistor M36 receives the same gate voltage from transistor M36, and so also operates in saturation to sink the output current I_OUT (equal to reference current Io1) from transistor M35. In this manner, cascode current mirror 300 provides proper current mirroring functionality.
The minimum output voltage of cascode current mirror 300 is determined by the gate voltages provided to cascoded transistors M35 and M36. As noted above, the voltage provided to the gate of transistor M35 is equal to the voltage at the gate of transistor M33. The voltage at the gate of transistor M33 is given by the following:Vg(33)=Vgs(33)+Vds(34)  (8)where Vgs(33) is the gate-source voltage of transistor M33 and Vds(34) is the drain-source voltage of transistor M34.
Transistor M34 is configured such that the when Vgs(33) is equal to Vdsat(33)+Vt(33), the voltage drop across transistor M34 is equal to Vdsat(33). As is known in the art, this is accomplished by sizing the W/L (width to length) aspect ratio of transistor M34 to be one-third of the W/L aspect ratio of transistor M33. Then, if transistors M31–M33 and M35–M36 are matched (i.e., have equal saturation voltages Vdsat and threshold voltages Vt), Equation 8 resolves to the following:Vg(33)=Vt+2Vdsat  (9)
This voltage is provided to the gate of transistor M31, which is also operating in saturation. Therefore, the gate-source voltage Vgs(31) of transistor M31 is equal to its threshold voltage (Vt) plus its saturation voltage (Vdsat). The source voltage Vs(32) of transistor M32, which is equal to the actual voltage at the gate of transistor M31 minus the gate-source voltage of transistor M31, is therefore simply equal to saturation voltage Vdsat.
Because transistor M36 is gate-coupled to the gate of transistor M32, the source voltage of transistor M36 is also equal to Vdsat. Meanwhile, the drain-source voltage of transistor M35 can swing down to its saturation voltage Vdsat before it falls out of saturation. Therefore, the minimum output voltage of cascode current mirror 300 is twice saturation voltage Vdsat (i.e., 2Vdsat).
Thus, cascode current mirror 300 provides an improved output voltage swing over Wilson current mirror 200 (shown in FIG. 2) while maintaining a high output impedance. However, the added complexity of current mirror 300 (i.e., the additional control branch formed by current source CS32 and transistors M33 and M34) can have undesirable cost, die area, and power consumption consequences.
Accordingly, it is desirable to provide a simple current mirror circuit that provides a wide output voltage range with a high output impedance.